TCDD (2,3,7,8-terachlorodibenzo-p-dioxin) is one of the most toxic substances known. In the guinea pig the LD50 appears to be less than 1 part per billion body weight. The compound is present at low concentrations as an impurity in some industrial chlorophenols and their derivatives. Because of its chemical stability and its greater solubility in lipids than in water, it is likely that TCDD undergoes biomagnification, concentrating as it moves up the food chain. The lowest limit of detection as yet reported for TCDD is 50 parts per billion. Thus, for adequate environmental monitoring, there is a need for increasing the sensitivity of the analytical method by a large factor. This project would complete the development of a technique for the reliable detection and identification of TCDD at levels of the order of 10 to the minus 3rd power parts per billion. The procedure involves chemical clean-up and specially designed gas-liquid chromatography followed by mass-spectroscopy. The output of the mass-spectrograph is placed in a time averaging computer, achieving a considerable improvement in signal-to-noise ratio. Isotopic dilution is used to monitor recovery by the addition of a known amount of Cl37 isomer of TCDD to each sample before analysis. When the technique is perfected, it will be applied to representative samples from the human food chain.